Issue with subfloor replacement

Hi,
When remodling our bathroom, we had to romove a big part of the subflloor because of water damage and also because we had to move the toilet. The issue is that the part we removed goes beyond a parition wall. My husband's idea is to nail a joist under the partition wall to support both the new subloor in the bathroom and the remaing part of the old one (closet in adjacent room). I am not sure if this is good for the structure and if this will lead to problems in the long run. Could you please advice. Thanks a lot.

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From your description, it does sound like it is a partition wall, runs perpendicular to joists?? If yes, what you propose sounds good to me. But use something larger 2x6 or same size as joists. All plywood joints not supported, should also be.

Thanks a lot for your quick reply.
Here a little drawing which might clarify the situation:
The subfloor was running under the wall to the joist in the bedroom. Now my question is whether it is sufficient to screw a 4x2 (or another size) from to the bottom of the wall frame on which I could screw the plywood covering the now cut out area.
Would sistering the joist next to the wall in the bathroom help?